Two of the most important applications of microwave technology include microwave communications systems and radio detection and ranging (radar).
Microwave (or radio frequency (RF)) communications systems can be used to provide communications links to carry voice, data, or other signals over distances ranging from only a few meters to deep space. At a top-level, microwave communication systems can be grouped into one of two types: guided systems, where the signal is transmitted over a low loss cable or waveguide; and radio links, where the radio signal propagates through space. In a broad sense, radio link microwave communications systems and radar systems operate in a similar way and share many components.
Developed during World War II, radar is quite possibly the most prevalent application of microwave technology. In the basic operation of radar, a transmitter sends out a signal, which is partially reflected by a distant target, and then a sensitive receiver detects the partially reflected signal. If a narrow fixed beam antenna is used, the direction of the target can be accurately given by the position of the antenna beam. The distance to the target is determined by the time required for the transmitted signal to travel back to the receiver after reflecting off of the target. The radial velocity of the target can be determined from the Doppler shift of the reflected return signal. Radar systems can be used in a variety of applications, including airport surveillance, aircraft landing, marine navigation, weather radar, meteorological surveillance, speed measurement (i.e., police radar), detection and tracking of aircraft, missiles and spacecraft, missile guidance, fire control for missile and artillery, astronomy, mapping and imaging, and the remote sensing of natural resources.
An important component in radar and radio link communication systems is the antenna. An antenna is a component that converts a wave propagating on a transmission line to a wave propagating in free space (transmission), or a wave propagating in free space to a wave propagating on a transmission line (reception). A wide variety of antenna types and geometries exist, including aperture antennas, reflector antennas, phased array antennas and combinations thereof.
Aperture antennas are often flared sections of waveguide, typically referred to as a horn, or simply even an open ended waveguide. Such antennas are commonly used at microwave frequencies and have moderate antenna gains. Antennas of this type are often used for aircraft and spacecraft applications, because they can be conveniently flush mounted on the skin of the vehicle and filled with a dielectric material to provide protection to the aperture from hazardous conditions of the environment, while maintaining the aerodynamic properties of the vehicle.
Reflector antennas are typically used for applications requiring high antenna gains, such as radar systems. Usually, the high gains of such antennas are achieved by focusing the radiation from a small antenna feed onto an electrically large reflector. An antenna feed is a component of an antenna that couples electromagnetic energy (i.e., microwaves or radio waves) to a focusing component of an antenna structure, such as a reflector. In other words, for transmission, an antenna feed guides RF energy from a transmission line to a reflecting or directive structure that forms the RF energy from the antenna feed into a beam or other desired radiation pattern for propagation in free space. For reception, an antenna feed collects incoming RF energy, which is converted it into RF signals that are propagated along a transmission line to the receiver. Often, the antenna feed is a dipole, horn or even open ended waveguide. Antennas typically consist of a feed and additional reflecting or directive structures (such as a parabolic dish or parasitic elements) whose function is to form the radio waves from the feed into a beam or other desired radiation pattern.
The high gains provided by reflector antennas are useful for increasing the range of a microwave system. Reflector antennas, of which dish antennas are a specific type, are relatively easy to fabricate and are typically quite rugged. However, such antennas can be large and unwieldy to move. Because of this, robust mechanical systems are typically needed to steer reflector antennas. The directive beam of reflector antennas are typically directed along the boresight axis and steered solely by mechanical means.
Phased array antennas are comprised of multiple stationary antenna elements, typically identical, which are fed coherently and use variable phase or time delay control at each element to scan a directive beam to a given angle and space. (Variable amplitude control is often also used to provide beam pattern shaping.) Examples of typical phased array antenna elements, also called radiators, include dipoles, microstrip or patch elements. The primary advantage that a phased array antenna has over more traditional antenna types, such as aperture and reflector antennas, is that the directive beam that can be repositioned, i.e., scanned, electronically. Electronic beam steering can be useful for quickly and accurately repositioning a beam.
Hybrid antennas, such as reflector antennas with a phased array feed, combine useful characteristics of both antenna types, such as the high gain and robust design of a reflector antenna and the agile electronic capabilities of a phased array antenna. Although not typically used due to design costs outweighing the increased performance, a hybrid reflector antenna with a phased array feed can be electronically scanned over a limited angular region.
Many microwave systems, such as high-power radar systems, rely on waveguide transmission lines for the low loss transmission of microwave power. Waveguide, which is typically a rectangular or circular tube, is capable of handling high power microwave signals but is bulky and expensive. Because waveguides are comprised of a single conductor, they support transverse electric (TE) and transverse magnetic (TM) waves, which are characterized by the presence of longitudinal magnetic or electric field components, respectively.
Waveguides were one of the earliest types of transmission lines used to transport microwave signals and are still used today. Because of this, a large selection of waveguide components, such as splitters/combiners, couplers, detectors, isolators, attenuators, phase shifters and slotted lines, are commercially available for various standard waveguide bands from 1 giga-hertz (GHz) to over 220 GHz. Due to the recent trend towards miniaturization and integration, many microwave circuits are currently being fabricated using planar transmission lines, such as microstrip and stripline, instead of waveguide. However, there is still a need for waveguide in many applications that require high power, such as high-power radar and millimeter wave systems.
For the sake of design simplicity, most waveguide-based transmission line systems support only a single “fundamental” propagating mode. Although waveguide is generally considered a low-loss type of transmission line, ohmic losses can significantly limit the distance over which energy traveling in the fundamental mode can be transmitted. Due to the inverse relationship between wavelength and frequency, high-frequency waveguide components, such as millimeter wave systems, have small dimensions. In high-power systems, voltage breakdown, or arcing, can occur when the dielectric material (typically air for waveguides) separating conductors breaks down. Such arcing is more likely to occur in high-power, high-frequency systems because of the relatively small dimensions between conductors.
To avoid these limitations, particularly in applications requiring high power and high frequency signals, overmoded waveguide is useful. “Overmoded” refers to waveguide structures where the dimensions are larger than the wavelength of the transmitted signal. In such waveguide geometries, more than one propagating mode can simultaneously exist. Such waveguide geometries can be useful to significantly reduce ohmic loss by propagating a particular mode, wherein the electric and magnetic fields maximum are situated far from the walls (i.e., the conductor) of the waveguide. The power saved by avoiding ohmic loss by using overmoded waveguide can be offset by unwanted mode conversion, where power can be shifted from the intended mode to a parasitic mode. Such parasitic mode conversion typically results in power loss and reflections due to mismatch.
For highly oversized waveguide, many propagating modes can exist. One of these modes can be selected for efficient, low loss transmission in a radar transmission line. Typically, such a mode is low order and couples well with a free-space radiating beam, i.e., the low order mode is well-matched to the propagation coefficient of free-space. In such instances, the propagating mode represents the beam pattern at the feed horn which illuminates the radar's focusing antenna. Generally, the goal is to have a pure, single mode at the feed to minimize beam distortion.